The handling of combustible materials, while necessary in many industrial processes and facilities, often presents a danger because of the potential for combustible materials to combust, detonate, or explode in process equipment, thereby damaging the equipment and injuring personnel. Combustible materials are often present in the form of liquids, gases, or both, in the reactors, conduits, storage vessels, separations apparatus, etc. of various manufacturing processes, such as those which produce (meth)acrylates, (meth)acrcylate esters, nitrites, and various other materials.
When combustible gas is present in sufficient concentrations, by volume, with oxygen, exposure to an ignition source, such as a spark, a chemical reaction, a hot wire or other hot surface, or even a hot spot (i.e., a localized region of increased temperature in a body of fluid) may initiate localized deflagration in the combustible gas. Combustible gases generally deflagrate at concentrations above the lower explosive limit and below the upper explosive limit of the combustible gas. Combustible gas may, of course, be comprised of a single kind of gas, but is often a mixture of one or more different gases, the particular composition of which renders the mixture combustible. Where a combustible gas is present, there is always some risk of deflagration because inadvertent ignitions cannot be 100% prevented.
The deflagration may or may not progress, ultimately, to one or more further damaging events such as detonation or explosion. Of course, such events have the potential to cause severe damage to equipment, as well as serious injury, or even death, to personnel who operate and monitor the equipment. Thus, the control, containment and suppression of ignition, combustion, deflagration and detonation or explosion of combustible materials are of critical importance to the safe and ongoing operation of many industrial manufacturing and processing operations. While some apparatus and technologies address avoidance of ignition, others attempt to contain, suppress, and extinguish a combustion event which would otherwise develop following inadvertent ignition. See, e.g., Grossel, Stanley S., Ed., Deflagration and Detonation Flame Arresters, AlChE (Center for Chemical Process Safety), New York, N.Y. (2002), Chapter 5, pp. 77-138.
For instance, U.S. Pat. No. 3,356,256 discloses the use of heat-dissipating metal netting within fuel storage containers as a means to absorb heat within the container that might otherwise lead to ignition. U.S. Pat. No. 4,613,054 discloses the use of aluminum foil balls with high thermal conductivity to accomplish the same result. Both approaches seek to employ a heat-sink to prevent sustained combustion from being achieved.
Similarly, Fauske (See, Fauske, Hans K., Expanded-Metal Networks: A Safety Net to Thwart Gas Explosions, Chemical Engineering Progress, December 2001, pp. 66-71) proposes the use of expanded metal foils as heat sinks within storage tanks to prevent flame-front propagation by removing combustion zone heat-energy. Fauske mimics the design of deflagration arrestors by employing high surface area (400 m2/m3 and higher) metal foils with tight passages for flame quenching. In relatively static storage tanks, such tight passages are of little consequence. However, in process vessels, such as reactors, the flow of process fluids through tight passages raises pressure drop across the vessel and consequently increases motive-force energy consumption. Additionally, in order for metal foil networks to effectively quench combustion (and thereby avoid subsequent progression to detonation or explosion), a large enough temperature differential (?T) must be maintained such that heat energy from the combustion zone can be quickly transferred into the metal foil. Unfortunately, many process vessels used to handle, react or store combustible gases are operated under conditions of elevated temperatures and/or pressures. As is well known in the art of hydrocarbon processing, increased heat and pressure both widens flammability limits (greater range of composition supports combustion) and reduces the incremental energy required to maintain self-sustaining combustion. Thus in many combustible gas process vessels, such as oxidation reactors, the use of heat-dissipating components, such as expanded metal foils, is largely ineffective at quenching combustion. Propagation of the deflagration pressure wave and flame front in such high temperature and pressure process vessels are simply too fast for heat absorption to occur at a rate that is sufficient to quench the combustion.
U.S. Pat. No. 5,495,893 discloses an apparatus and method to control deflagration of gases, wherein the apparatus includes a combustible substance detector which triggers delivery of a deflagration suppressant into the combustible substance, by a liquid atomizing device for controlling the size of the liquid suppressant droplets. The disclosure of this patent states that deflagration can be effectively suppressed by heat absorption, such as by utilizing a fine mist liquid stream (i.e., the deflagration suppressant) that can be rapidly vaporized to quickly remove the heat by which a deflagration propagates. This solution is based on the understanding that, in a deflagration, the combustion of a combustible gas initiates a chemical reaction that propagates outward by transferring heat and/or free radicals to adjacent molecules of the combustible gas. The transfer of heat and/or free radicals ignites the adjacent molecules and, in this manner, the deflagration propagates or expands outward through the combustible gas.
U.S. Pat. No. 6,540,029 discloses a deflagration suppression and explosion isolation system which has the goal of suppressing the deflagration stage of an explosion and preventing deflagration phenomena originating in a containment structure from propagating into an associated conduit and then transitioning into detonation phenomena in the conduit. The system described in this patent includes a pressure detector for detecting a rapid rise of pressure, which is indicative of an incipient explosion, and a suppressant device which directs a fire suppression agent into the combustible gases, as well as a gate valve assembly which closes in tandem with release of the suppressant agent to redirect the flame and combustion generated pressures. Again, the device in U.S. Pat. No. 6,540,029 operates to introduce an additional material, a suppressant agent, into the combustible gas for absorption of heat which otherwise facilitates propagation of the deflagration and its transition to detonation. This patent acknowledges that, during a deflagration event, a pressure wave and a flame front are generated at the point of ignition and propagate outward in all directions therefrom, with the pressure wave traveling faster than the flame front. Furthermore, obstacles and bends in a pipe or conduit containing the combustible gas will increase turbulence (i.e., mixing) which, in turn, accelerates the transition from deflagration to detonation. As is well understood in the art, increases in turbulence and mixing of combustible gas are to be avoided when implementing deflagration control measures.
In Razus D., et al., Closed vessel combustion of propylene-air mixtures in the presence of exhaust gas, Fuel (2007), doi:10.1016/j.fuel.2006.12.009, it is recognized that characteristic parameters of explosion propagation in closed vessels include the peak pressure reached after deflagration, the time required to reach that peak pressure and the decrease of peak pressure achieved by introduction of diluent material into a combustible gas after ignition. This research article concludes that, exhaust gas, which typically contains carbon dioxide and water vapor, has an important inerting effect on flammable fuel-air mixtures and may be considered a cheap diluent for mitigation of fuel-air explosions. While this technology addresses controlling or minimizing an explosion after ignition of combustible gas, it does so by adding an inert material, which is similar to the method of introducing flame suppressants to a combustible gas after ignition to absorb heat and slow propagation of the flame front during deflagration.
The technology described in U.S. Pat. No. 6,932,950 is an attempt to minimize or eliminate secondary reactions, including but not limited to ignition and deflagration, which have greater risk of occurring at the inlet side of a tubular reactor where the feed gas mixture is a combustible gas coming in contact with the hot spots proximate to the inlet tube-sheet of the reactor. This patent discusses the previously known practices of placing a layer of ceramic materials, or wire mesh, in the inlet chamber, adjacent to the inlet tube sheet on the inlet gas side, but not filling more than about 20% of the chamber's volume, to create a barrier between the incoming feed gas and the hot tube-sheet. Other methods described include creating a cooling chamber proximate to the hot tube-sheet (chamber is filled with circulating air), as well as forming a solid barrier (e.g., using poured resin material to form a layer adjacent to the tube-sheet, on the heat carrier side to insulate the tube-sheet from the heat carrier). The solution described in U.S. Pat. No. 6,932,950 is the provision of a separate insulation chamber within the inlet chamber of the reactor that is proximate to and on the inlet side of the tube-sheet. The insulation chamber is sized and shaped to be commensurate with the cross-section of the tube-sheet, and is either evacuated or filled with air, sand, oil, or any other suitable solid, liquid, or gaseous material incapable of reacting with the heat carrier. All of these technologies are aimed at prevention of ignition and other undesirable side reactions by placement of a barrier of material between the hot tube-sheet and incoming gaseous feed streams. None of these technologies involve filling the gas inlet or outlet regions of the reactor with suitably shaped and sized attenuating material which deflects the pressure wave of a deflagration, after inadvertent ignition to quench and contain the deflagration and prevent it from progressing to detonation.
A device for physically diverting the flame front of a deflagration event is described in U.S. Pat. No. 7,000,630, wherein a flame front diverter directs the high-speed pressure wave towards a bi-directional rupturable disc causing the rupturable disc to open thus creating an aspiration effect on the opposite bi-directional rupturable disc, which creates an escape path for the pressure wave and flame front, as well as drawing in fresh air as a diluent. The flame front diverter described in this patent is designed to prevent a deflagration from propagating from one vessel to another.
There remains a need for an effective, simple and economical method for controlling, containing and suppressing explosion of combustible gas in a process vessel. Applicant have developed a method and apparatus which addresses this need by attenuating, and thereby diminishing, the pressure wave created during a deflagration to prevent the deflagration from transitioning to detonation, rather than attempting to suppress and quench the flame front by removing heat from a combustible gas which has been ignited by providing solid materials known to absorb heat into the process vessel, or by introducing flame retardant or suppressants materials after ignition has occurred.